Method for the production of methylenedi(phenylamine and methylenedi(phenyl isocyanate)

ABSTRACT

A process for preparing methylenedianiline by reacting aniline with formaldehyde in the presence of acid catalysts comprising, in a semicontinuous process, introducing aniline with or without acid catalyst, feeding formaldehyde with or without acid catalyst through a mixing element into a circuit in which aniline with or without acid catalyst and with or without previously added formaldehyde is circulated and, after feeding in at least 50% of the total amount of formaldehyde to be fed in, heating the reaction mixture to a temperature above 75° C. In addition, the invention relates to a process for preparing polyisocyanates by phosgenation of amines obtainable in this manner and to the polyisocyanates obtainable by this process.

The present invention relates to a process for preparingmethylenedianiline by reacting aniline with formaldehyde in the presenceof acid catalysts, the mixtures which can be prepared by this processcomprising methylenedianiline, a process for preparing polyisocyanatesby phosgenation of these mixtures comprising methylenedianiline, andpolyisocyanates obtainable in this manner.

The preparation of methylenedianiline (also termed MDA below), isgenerally known and is customarily carried out by continuous orbatchwise reaction of aniline with formaldehyde in the presence of acidcatalysts. In this reaction, whose main product is 4,4′-MDA, theunwanted byproduct N-methyl-MDA is formed to a small extent. Thisbyproduct is disadvantageous, in particular in the subsequent reactionof the MDA with phosgene to prepare methylenebis(phenyl isocyanate),also termed MDI, since the N-methyl-MDA is the precursor compound forchlorinated byproducts in the MDI and chlorine contents in the MDI aslow as possible are sought.

To decrease N-methyl-MDA as byproduct in the preparation of MDA, variousprocesses are known.

Thus, U.S. Pat. No. 5,286,760, for continuous MDA preparation, describespartial neutralization of the reaction mixture between the stage ofcondensation of two molecules of aniline and one molecule offormaldehyde and the subsequent rearrangement of the intermediateaminobenzylamines, abbreviated as ABA, to give MDA. EP-A 451 442 andDD-A 238 042 disclose, for a continuous process, the addition offormaldehyde over a plurality of process stages. Processes fordecreasing the byproduct are also known for batchwise processes. DD-A295 628 describes the addition of formaldehyde in two steps during thecondensation stage, in the first addition the main amount offormaldehyde being added at a low temperature and the second addition ofthe remaining formaldehyde being performed at the same or highertemperature.

A disadvantage in these processes is the insufficient decrease of theN-methyl-MDA content in the product mixture, so that there is still aneed for improvement.

Processes for preparing MDI from MDA by phosgenation are generallyknown.

It is an object of the present invention to develop a process forpreparing methylenedianiline by reacting aniline with formalde-hyde inthe presence of acid catalysts which minimizes the N-methyl-MDA contentas an unwanted byproduct. Such an MDA should be used, in particular, inan improved process for preparing methylenebis(phenyl isocyanate) (MDI),which should make accessible an MDI having improved properties, inparticular a low chlorine content and/or a light color, in particular inthe crude MDI which, in addition to the monomeric MDI, also comprisespolymeric MDI, and/or should be made accessible in the polymeric MDI.

We have found that this object is achieved according to the invention,in a semicontinuous process, by introducing aniline with or without acidcatalyst, feeding formaldehyde with or without acid catalyst through amixing element into a circuit in which aniline with or without acidcatalyst and with or without previously added formaldehyde is circulatedand, after feeding in at least 50% of the total amount of formaldehydeto be fed in, heating the reaction mixture to a temperature above 75° C.

This novel procedure permits a higher content of higher MDA oligomers tobe obtained than is possible by a continuous procedure at high molarratios of aniline to formaldehyde without recycling the MDA. By theprocess according to the invention, minimizing the content of unwantedbyproducts is possible.

The reaction according to the invention of aniline with formaldehyde,preferably in the presence of acid catalysts, is performed according tothe invention semicontinuously, i.e. one reaction component, the anilineand preferably the acid catalyst, is introduced and the second reactioncomponent, the formaldehyde with or without acid catalyst, is added tothe first reaction component. Preferably, the process according to theinvention is carried out in such a manner that aniline and acid catalystare introduced and formaldehyde is added to this first reactioncomponent. The reaction is customarily carried out at temperatures offrom 20 to 150° C. Preferably, the process according to the invention iscarried out in such a manner that the formaldehyde is added to thereaction mixture in the circuit, i.e. to the aniline, the acid catalystand to formaldehyde which has possibly been previously added andreaction products, up to an amount of at least 50% of the total amountof formaldehyde to be fed, preferably up to complete addition of all ofthe formaldehyde, at a reaction mixture temperature in the circuit offrom 20 to 75° C., preferably from 20 to 60° C., particularly preferablyfrom 30 to 40° C.

The temperature effects the isomeric distribution of themethylenedianiline in the product. If, preferentially, 2,2′-and/or2,4′-methylenedianiline are to be prepared, a high temperature may beadvantageous. The reaction mixture can be heated by generally customarydevices, e.g. by heat exchangers in the pumped circuit or a secondpumped circuit and/or via the reactor wall.

The reaction mixture, after feeding into it at least 50% of the totalamount of formaldehyde to be fed, is, preferably towards the end of thefeed of formaldehyde solution, particularly preferably after thecomplete addition of the entire amount of formaldehyde to the reactionmixture, heated, preferably for a period of at least 0.2 hours,particularly preferably from 0.2 to 48 hours, in particular from 0.2 to6 hours, at a temperature of above 75° C., preferably above 90° C.,particularly preferably from 105 to 150° C., especially from 110 to 135°C. Particularly preferably, after complete addition of the formaldehydeto the reaction mixture, the reaction mixture can be heated for a periodof from 0.1 to 120 minutes at a temperature of from 65 to 100° C. andthen, as described above, at a temperature of above 75° C. This heatingoffers the advantage that the handleability of the reaction mixture issimplified, since the reaction mixture has a lower viscosity at thehigher temperature. At the same time, during this heating, unwantedbyproducts in the reaction mixture are broken down or rearranged in anageing phase. The reaction mixture can be aged under these preferredconditions in the apparatus in which the reaction of formaldehyde withaniline was carried out, or else batchwise or continuously in anotherapparatus into which the reaction mixture can be transferred aftercomplete addition of the formaldehyde. For example, the reaction mixturecan be aged in the apparatus in which the formaldehyde solution is fedor was fed. It is also possible to pass the reaction mixture from theapparatus into at least one further reactor, for example a tubularreactor and/or stirred tank, and to perform the ageing in this reactor(these reactors) at a temperature of above 75° C. Preferably, thereaction mixture, after complete addition of the formaldehyde, istransferred to another apparatus in which the ageing is completed.Particularly referably, the reaction mixture, after complete addition ofthe formaldehyde which took place preferably at a temperature of from 20to 60° C., particularly preferably from 30 to 40° C., is transferredinto a customary storage vessel, heated as described preferably at atemperature of from 65 to 100° C. and then heated in conventionalreactors, preferably a tubular reactor, as described preferably at atemperature of from 105 to 150° C., particularly preferably from 110 to135° C.

The reaction mixture can thus be passed into, for example, tubularreactors, stirred tanks, stirred tank cascades, combinations of stirredtanks and tubular reactors in which the reaction to give MDA can becompleted.

The reaction mixture comprising MDA and customarily polymeric MDA can beworked up after the reaction by generally known processes, for exampleby neutralization, phase separation, distillation and/or chromatographicseparation methods, preferably by neutralization, preferably at from 60to 110° C., and removal of water, aniline and possibly other unwantedminor components by distilling these substances.

Preferably, the reaction mixture is neutralized, preferably with aqueoussodium hydroxide solution, for example 50% strength aqueous sodiumhydroxide solution, preferably at from 60 to 110° C., and the aqueousphase is then removed by phase separation. To remove inorganicimpurities, the organic phase can be washed at customarily from 60 to110° C. with water, the aqueous phase can be separated off and thenunreacted aniline can be removed from the organic phase, that is to saythe MDA, by distillation, preferably at a pressure of from 1050 to 5mbar and a preferred temperature of from 180 to 240° C.

The starting components formaldehyde, aniline and acid catalyst can beused at customary purities, the formaldehyde being able to be inequilibrium with higher molecular weight addition products such aspoly(oxymethylene)glycols. The formaldehyde can be used in customary,for example aqueous, solutions having a formaldehyde content of from 10to 60% by weight, based on the weight of the solution. The formaldehydecan also be fed in the gaseous state. In this case, it is fed as puregas or as a mixture with inert gas. If required, water can be addedseparately.

The reaction mixture can be circulated in a suitable apparatus bygenerally customary devices, for example pumps. The rate at which thereaction mixture is circulated is preferably from 1 to 6 m/sec. Theformaldehyde solution can be fed via a reaction mixing pump, such asdescribed in DE-A 4220239 or via a nozzle system, e.g. a ring-gapnozzle, built into the pump circuit. In the case of the reaction mixingpump, the device not only serves for feeding in the formaldehyde andpreferably complete mixing, but also for moving the reaction mixture inthe apparatus. If a nozzle is used, the reaction mixture can be moved inthe apparatus by conventional pumps known in chemistry. The mixingenergy dissipated locally during the feed of formaldehyde into thereaction mixture in the mixing zone of the mixing element, i.e. forexample the nozzle or the reaction mixing pump, is preferably from 100to 100,000 W/l. The quantity in the pumped circuit is in a ratio to thequantity of formaldehyde solution fed into the circuit of preferably atleast 20:1.

As acid catalyst, use can be made of catalysts generally known for thisreaction, for example acids having a pKa<1.5, e.g. mineral acids such asphosphoric acid, sulfuric acid and/or hydrochloric acid (HCl);preferably HCl is used. Aniline and the acid catalyst, preferably HCl,are preferably mixed at from 30 to 60° C, preferably from 35 to 45° C.

The molar ratio of aniline to acid catalyst in the reaction mixture iscustomarily from 1:0.6 to 1:0.01, preferably from 1:0.3 to 1:0.05. Thismolar ratio applies in particular to the particularly preferredembodiment in which aniline and acid catalyst are introduced and thenformaldehyde and no further acid catalyst is added.

The molar ratio of aniline to the total amount of formaldehyde to beadded is customarily from 1.7:1 to 7.2:1, preferably from 1.9:1 to5.1:1, particularly preferably from 1.9:1 to 3.6:1. The formaldehyde ispreferably fed into the circuit through a nozzle or a reaction mixingpump. In order to avoid unwanted parallel reactions leading tobyproducts, the formaldehyde is preferably added in such a manner thatas rapid and complete mixing as possible takes place with the reactionmixture which is situated in the apparatus. This can be achieved, forexample, by generating a turbulent flow in the mixing chamber. In theprocess according to the invention, preferably in one apparatus, anilineand preferably HCl as acid catalyst are introduced, mixed, circulated,for example by a connected conventional pump, and formaldehyde is addedto this reaction mixture, preferably via a reaction mixing pump ornozzle. The formaldehyde can be added in such a manner that constantvolumes per unit time are fed into the reaction mixture until there is asuitable molar ratio of aniline to formaldehyde in the reaction mixture.Preferably, the addition is performed in such a manner that, per minute,from 0.05 to 2% of the original volume of the aniline in the apparatusare passed as volume of formaldehyde solution into the reaction mixture.Instead of introducing a constant volume of formaldehyde per unit time,the formaldehyde can be added to the reaction mixture in such a mannerthat the volume of the formaldehyde added per unit time decreases inaccordance with a mathematical function as the addition progresses.Preference is given to an addition rate which is constant, fallinglinearly, or falling in stages. Furthermore, the formaldehyde can beintroduced in pulses into the reaction mixture, in which case a regularor irregular pulse frequency and addition rate can be selected. Thetotal amount of formaldehyde to be introduced should preferablycorrespond to the molar ratios described at the outset in relation tothe amount of aniline. In this batchwise procedure, the reaction mixtureis emptied from the apparatus after the desired conversion rate andfurther worked up if necessary.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic view of an apparatus in which the reaction of thepresent invention may be carried out.

FIG. 2 is a schematic view of another apparatus in which the reaction ofthe present invention may be carried out.

DETAILED DESCRIPTION OF THE INVENTION

The reaction according to the invention can be carried out, for example,in an apparatus which has

1: feed lines for aniline and acid catalyst,

2: feed line for formaldehyde,

3: at least one mixing element, for example a reaction mixing pump ornozzle through which the formaldehyde is fed into the apparatus,

4: at least one reactor having

5: optional devices for mixing the reaction mixture,

6: a pipe system which, starting from the reactor, makes circulation ofthe reaction mixture possible,

7: a device for heating the reaction mixture and

8: an optional pump which circulates the reaction mixture in (6) and

9: at least one connection for taking off the reaction mixture.

An apparatus of this type is shown in FIG. 1 by way of example, in whichfigure it may be noted that aniline and acid catalyst can be addedeither together, as shown in FIG. 1, or separately, at substantially anypoint of the apparatus, for example by addition to the reactor (4) orthrough connections to the reaction mixing pump or nozzle (3). Thedevices, 7, 8 and, in particular, 9, can also be disposed substantiallyanywhere, for example, in the case of the connection 9, on the reactor 4as well.

The selected capacity of the reactor (4) can vary depending on thedesired conversion rate. The selected diameter, which can also vary, andthe length of the pipe system (6) can also vary substantially as desireddepending on batch size. For components (1) to (9) conventional devicescan be used, as already described for components (3) and (7). Anapparatus suitable for carrying out the process according to theinvention can consist of materials customary for this purpose, forexample steel/enamel or stainless steel alloys.

The process product, customarily also termed crude MDA, i.e. the mixturecomprising methylene- dianiline, for example 2,2′-, 2,4′-, and/or4,4′-MDA as monomeric MDA, and customarily polymeric MDA, also referredto as polymethylenedianiline, preferably comprises less than 0.09% byweight of N-methyl-MDA and is preferably used for the known synthesis ofmethylenebis(phenyl isocyanate), known as MDI ordiphenylmethanediisocyanate, for example 2,2′-, 2,4′- and/or 4,4′-MDIand polymeric MDI, for example by conventional phosgenation ofpolyamines.

The phosgenation can preferably be carried out in one or more steps incustomary, particularly preferably inert, solvents, e.g. chlorinatedaromatic hydrocarbons, for example monochlorobenzene, dichlorobenzenessuch as o-dichlorobenzene, p-dichlorobenzene, trichlorobenzenes, thecorresponding toluenes and xylenes, chloroethylbenzene,monochlorodiphenyl, alpha- or beta-naphthylchloride and dialkylphthalates, such as diethyl isophthalate, preferably toluene, mono-and/or dichlorobenzene, in conventional reactors, for example stirredtanks, stirred tank cascades, columns and/or tubular reactors at knowntemperatures of, for example, from 50 to 150° C., preferably from 70 to120° C., particularly preferably from 70 to 100° C. and at a pressure offrom 0.5 to 10 bar, particularly from 0.8 to 5 bar, particularlypreferably from 0.8 to 1.5 bar.

For example, the phosgenation can be carried out by a two-step reactionin the presence of at least one inert orgnaic solvent, the firstphosgenation step being carried out in a static mixer and the secondphosgenation step being carried out in a dwell-time apparatus, and inthe dwell-time apparatus the mass ratios of phosgene to hydrogenchloride being at the same time 10-30:1 in the liquid phase and 1-10:1in the gas phase.

Static mixers which can be used for the first phosgenation step are theknown and abovementioned apparatuses, in particular nozzles. Thetemperature in the first phosgenation step is customarily from 50 to120° C., preferably from 60 to 120° C., particularly preferably from 90to 120° C.

The mixture of the first phosgenation step is preferably fed to adwell-time apparatus, according to the invention the mass ratios ofphosgene to hydrogen chloride in the dwell-time apparatus of the secondphosgenation step being at the same time 10-30:1 in the liquid phase and1-10:1 in the gas phase.

Dwell-time apparatuses which can be used for the process of theinvention are known apparatuses, preferably stirring machines, inparticular stirred-tank cascades having from 2 to 6 stirred tanks, ortowers, in particular those having <10 theroetical plates.

When stirring machines are used as dwell-time apparatuses, as mentionedabove, in particular stirred-tank cascades having at least 2, preferablyfrom 2 to 6, particularly preferably from 2 to 5, stirred tanks areused. In principle, a cascade having more than 6 stirred tanks can alsobe used, but increasing the number of stirred tanks above 6 onlyincreases the equipment required without any measurable improvement inthe end product occurring. The mixture of the first phosgenation stepcustomarily enters the first stirring machine at a temperature of70-120° C., preferably 85-105° C. The temperatures in the stirringmachines are preferably, jointly or differing individually, 75-120° C.,particularly preferably 80-110° C. The pressures in the stirringmachines are customarily individually differing or jointly 1.0-3.0 atm(gauge), preferably 1.2-2.5 atm (gauge).

Particularly preferably, a tower is used as dwell-time apparatus. Inthis case it is particularly advantageous to operate the tower incounter-current. The product mixture of the first phosgenation step ispreferably fed into the tower in such a manner that monomeric MDI/withor without polymeric MDI/solvent/phosgene mixture leaves the tower viathe bottom and a phosgene/hydrogen chloride mixture is taken off fromthe tower overhead and is fed to the hydrogen chloride/phosgeneseparation. The inlet temperature of the first phosgenation step mixtureinto the tower can preferably be 80-120° C., particularly preferably82-117° C. The bottom temperature of the tower is preferably 80-120° C.,particularly preferably 90-110° C. The top pressure of the tower ispreferably 1.0-4.7 atm (gauge), particularly preferably 2.0-3.7 atm(gauge). The hydrogen chloride/phosgene ratio in the tower is preferablyset and controlled by the phosgene excess in the first phosgenationstep, the reaction product inlet temperature into the tower, the towerpressure and the bottom temperature of the tower. The amount of phosgenecan be fed completely to the first phosgenation step, or only in part,in this case a further amount being fed into the dwell-time apparatus ofthe second phosgenation step. The tower used preferably has <10theoretical plates. The preferred use of a valve-tray tower isadvantageous. Other tower internals are also suitable which ensure thenecessary dwell time for the carbamyl chloride cleavage and rapid andeffective removal of hydrogen chloride, for example bubble-cap traytowers, distillation trays having deepened liquid weirs. The perforatedtray tower proposed in DE-A 3 744 001 can meet the object of gentlecarbamyl chloride cleavage with rapid and effective removal of hydrogenchloride technically only highly inadequately.

The mixtures (crude MDI) prepared by the process of the invention whichcomprise diphenylmethane diisocyanates (monomeric MDI) and polyphenylenepolymethylene polyisocyanates (polymeric MDI) customarily have adiphenylmethane diisocyanate isomer content of from 30 to 90% by weight,preferably from 30 to 70% by weight, an NCO content of from 29 to 33% byweight, preferably from 30 to 32% by weight, based on the crude MDIweight, and a viscosity, determined as specified by DIN 51550 at 25° C.,of preferably a maximum of 2500 mPa.s, preferably from 40 to 2000 mPa.s.

The amount of solvent in the phosgenation is expediently such that thereaction mixture has an isocyanate content of from 2 to 40% by weight,preferably from 5 to 20% by weight, based on the total weight ofreaction mixture.

Phosgene can be used as such or diluted with gases which are inert underthe reaction conditions such as nitrogen, carbon monoxide etc. The molarratio of crude MDA to phosgene is expediently such that from 1 to 10mol, preferably from 1.3 to 4 mol, of phosgene are present in thereaction mixture per mol of NH₂ groups. In a two-step process, theamount of phosgene can be fed completely to the first phosgenation stepor, in part, can also be added to the dwell-time apparatus of the secondphosgenation step.

The crude MDI prepared by phosgenation can be purified by customaryprocesses, for example distillation. Preferably, in a first purificationoperation, phosgene with or without solvent can be removed, preferablysubstantially, particularly preferably completely, from the phosgenationreaction mixture, i.e. from the crude MDI. This purification step canpreferably be carried out by a stripping process. In a stripping processof this type, the crude MDI can be passed into one or more apparatuseshaving a large internal surface area and can be distributed onto itssurface, so that readily volatile components can escape. The apparatuscan be, for example and preferably, a falling-film or thin-filmevaporator or a packed column of suitable design. Inert gases can be fedin as stripping medium and/or vacuum can be applied over the apparatus.The temperatures during this stripping process are preferably below 210°C., particularly preferably from 50 to 190° C. Preferably, the desiredmonomeric MDI, for example 2,2′-, 2,4′- and/or 4,4′-MDI and/or mixturescomprising at least two of these isomers, are separated off by asuitable process, preferably by distillation, for example at pressuresof from 2 to 50 mbar, preferably from 2 to 20 mbar, and temperatures offrom 150 to 250° C., preferably from 180 to 230° C., and/or preferablyby crystallization, for example by fractional crystallization.

Particularly preferably, the crude MDI is purified by removing phosgene,HCl with or without solvent, for example in a previously describedstripping process, possibly under vacuum or with feed of inert gas, fromthe crude MDI at a temperature of <150° C., preferably from 50 to 149°C., after preferably complete removal of the phosgene, separating offsolvent with or without chlorine-containing compounds from theisocyanate at a temperature of ≦209° C., preferably from 150 to 209° C.,particularly preferably ≦109° C., especially from 150 to 190° C, forexample in a previously described stripping process, the purificationsteps being able to be carried out by the previously describedapparatuses. Subsequently the desired monomeric MDI, for example 2,2′-,2,4′- and/or 4,4′-MDI and/or mixtures comprising at least two of theseisomers, can be separated off by a suitable process, preferably bydistillation, for example at pressures of from 2 to 50 mbar, preferablyfrom 2 to 20 mbar, and temperatures of from 150 to 250° C., preferablyfrom 180 to 230° C., particularly preferably from 210 to 230° C. and/orpreferably by crystallization, for example fractional crystallization.The monomeric MDIs are thus preferably separated from the polymeric MDIby distillation and/or crystallization.

The monomeric MDI and/or the polymeric MDI is then conventionallystabilized with an antioxidant based on sterically hindered phenolsand/or with at least one aryl phosphite. The stabilizers are expedientlyused in an amount up to a maximum of 1% by weight, preferably from 0.001to 0.2% by weight.

Suitable antioxidants based on sterically hindered phenols are, forexample: styrenated phenols, that is to say phenols which contain a1-phenylethyl group in the 2- or 4-position or in 2-and 4- and/or6-position, bis[2-hydroxy-5-methyl-3-tert-butylphenyl]methane,2,2-bis[4-hydroxyphenyl]propane, 4,4′-di-hydroxybiphenyl, 3,3′-dialkyl-or 3,3′, 5,5′-tetraalkyl-4,4′-di-hydroxybiphenyl,bis[4-hydroxy-2-methyl-5-tert-butylphenyl] sulfide, hydroquinone,4-methoxy-, 4-tert-butoxy- or 4-benzyl-oxyphenol, mixtures of4-methoxy-2- or -3-tert-butylphenol, 2,5-dihydroxy-1-tert-butylbenzene,2,5-dihdyroxy-1,4-ditert-butylbenzene, 4-methoxy-2,6-ditert-butylphenoland, preferably 2,6-ditert-butyl-p-cresol.

Aryl phosphites which have proven useful are tri(alkylphenyl) phosphiteshaving from 1 to 10 carbons in the alkyl radical, for exampletri(methylphenyl), tri(ethylphenyl), tri(n-propylphenyl),tri(isopropylphenyl), tri(n-butylphenyl), tri(sec-butylphenyl),tri(tert-butylphenyl), tri(pentylphenyl), tri(hexylphenyl),tri(2-ethylhexylphenyl), tri(octylphenyl), tri(2-ethyloctyl-20 phenyl),tri(decylphenyl) phosphite and preferably tri(nonylphenyl) phosphite,and, in particular, triphenyl phosphite.

These purification processes offer the advantage thatchlorine-containing compounds which lead to adverse properties in thedesired isocyanate are removed from the isocyanate and at the same timethe formation of coloring components is suppressed. In particular, thecrude-MDI and, after separating off the monomers, that is to say 2,2′-,2,4′- and/or 4,4′-MDI, from the crude MDI, the polymeric MDI in thedistillation bottoms have according to the invention a light color and alow chlorine content.

The process according to the invention for preparing methylenebis(phenylisocyanate) can thus be carried out, in a semicontinuous process, byintroducing aniline and acid catalyst, the molar ratio of aniline toacid catalyst being from 1:0.6 to 1:0.01, feeding formaldehyde through anozzle or a reaction mixing pump into a circuit in which aniline andacid catalyst with or without previously added formaldehyde can becirculated at a temperature of from 20 to 75° C., after feeding in atleast 50% of the total amount of formaldehyde to be fed in, heating thereaction mixture for a period of at least 0.2 hours at a temperatureabove 75° C., the molar ratio of the aniline introduced to the totalamount of formaldehyde to be added being from 1.7:1 to 7.2:1,neutralizing the resulting methylenedianiline, separating off water andaniline, phosgenating the purified methylenedianiline at a temperatureof from 50 to 150° C. and a pressure of from 0.5 to 10 bar in thepresence or absence of inert solvents, removing phosgene, HCl andpossibly solvent, for example in a previously described strippingprocess, from the crude MDI at a temperature below 150° C. possiblyunder vacuum or feeding in inert gas, then separating off solvent withor without chlorine-containing compounds, for example in a previouslydescribed stripping process, from the isocyanate at a temperature of≦190° C. and then separating off the desired monomeric MDI, for example2,2′-, 2,4′- and/or 4,4′-MDI and/or mixtures comprising at least two ofthese isomers, by a suitable process, preferably by distillation, forexample at pressures of from 2 to 50 mbar, preferably from 2 to 20 mbar,and temperatures of from 150 to 250° C., preferably from 180 to 230° C.,and/or preferably by crystallization, for example fractionalcrystallization.

The MDA and/or the polymeric MDA, for example the crude MDA, can bestored before the phosgenation at a temperature of from 100 to 130° C.

The polyisocyanates prepared using the methylenedianiline according tothe invention have the advantage, in particular, that they possess a lowhydrolyzable chlorine content. In addition, the isocyanate preparedaccording to the invention has a color which is desirably very light.These advantages are not only due to the preparation according to theinvention of the methylenedianiline having the low byproduct content,but are also due to the fact that the phosgenation of the amines and theproduct workup are carried out at low pressures and thus lowtemperatures. This defined combination of many process parametersbeginning with aniline to the final bis(isocyanate) leads to theparticularly advantageous products according to the invention.

Preferably, the isocyanates and polyisocyanates, for example crude MDI,monomeric MDI and polymeric MDI, particularly crude MDI, especiallypolymeric MDI, attainable according to the invention, have ahydrolyzable chlorine content of <0.1%, particularly preferably <0.045%,and an iodine color index of <30, particularly preferably <11, at adilution of 1:5 in monochlorobenzene.

The examples illustrate the invention.

COMPARATIVE EXAMPLE 1

The reaction was carried out in an apparatus which consisted of astirred-tank cascade having three reactors which had capacities of 700,800 and 800 ml, and a packed tube. The reaction temperatures in thereactors were set at 40 (first stirred tank), 70 (second stirred tank),80 (third stirred tank) and 120° C. (tubular reactor) by externalcooling and/or heating. The packed tube had a total volume of 5000 mland an internal tube diameter of 30 mm. The agitator speed in thereactors of the stirred-tank cascade was in each case 500 rpm. 1264 g/hof aniline, which had previously been mixed with 422 g/h of 30% strengthaqueous hydrochloric acid, were added to the first reactor. At the firstreactor was situated an external pumped circuit having a static ordynamic mixer into which 341 g/h of a 50% strength formaldehyde solutionin water were added by a pump. The product mixture from the tubularreactor was neutralized using sodium hydroxide solution. Phaseseparation was then performed at a temperature of from 70 to 80° C. Theorganic phase was separated off and washed with 1.5 times the volume ofwarm water. Excess aniline was distilled off from this purified phaseunder reduced pressure and recirculated to the first reactor. 24 h afterstarting up the plant, the reaction mixture was in a steady state andsamples of the organic phase were taken. The N-methyl-MDA content in theresulting product was 0.26% by weight. This polyamine was reacted in twostages with phosgene in a conventional process for preparingisocyanates. The hydrolyzable chlorine content in this polyisocyanatewas 0.22%.

EXAMPLE 1

An apparatus as shown in FIG. 2 was employed. In this FIG. 2, thereference numbers designate the following:

1: reactor 2: reservoir tank, feed of aniline and HCl 3: reactionmixture circulation circuit 4: reservoir tank, feed of formaldehydesolution 5: metering pump 6: mixing element, formaldehyde solutionadmission 7: pressure gage 8: flowmeter 9: heat exchanger 10: agitator11: temperature measurement 12: stopcock

The reactor 1 had a capacity of 1000 ml. The agitator speed was 500 rpm.The external circulation 3, reaction mixture circulation rateapproximately 130 l/h, was operated by a pump. 735 g of aniline wereintroduced from the reservoir tank and mixed with 243 g of 30% strengthaqueous hydrochloric acid in reactor 1. At a temperature of 40° C., atotal of 204 g of a 50% strength solution of formaldehyde in water wasthen added within one hour at a constant metering rate to the circuitvia the mixing element 6, a dynamic mixer. Directly after the additionof the formaldehyde solution, the reaction mixture was heated and thenkept at 120° C. for 2.5 hours. The reaction mixture was worked up asdescribed in comparative example 1. The N-methyl-MDA content in theresulting product was 0.07% by weight. This polyamine was reacted withphosgene in a two-stage process in the process according to theinvention for preparing isocyanates. The hydrolyzable chlorine contentin this polyisocyanate was 0.06%.

EXAMPLE 2

The procedure of Example 1 was followed, but the formaldehyde solutionwas added in a staged manner. In the first 30 minutes of the addition,the formaldehyde solution was metered into the reaction mixture at arate of 306 g/h, and in the second 30 minutes at a rate of 102 g/h. Thereaction mixture was worked up as described in Example 1. TheN-methyl-MDA content in the resulting product was 0.08% by weight. Thispolyamine was reacted with phosgene in a two-stage process in a processfor preparing isocyanates at a temperature of 80° C. and a pressure of 1bar. The hydrolyzable chlorine content in this polyisocyanate was 0.07%.The iodine color index of the isocyanate was 15 at a dilution of 1:5with monochlorobenzene.

The object of developing a process by which the undesired formation ofN-methyl-MDA is prevented, could thus be achieved by the processaccording to the invention. Not only was the content of undesiredN-methyl-MDA markedly decreased by 73 or 69%, but also the hydrolyzablechlorine content in the polyisocyanate which was produced using the MDAprepared according to the invention was drastically reduced by >70%. Theobject of preparing an isocyanate as light as possible starting from MDAwas also achieved.

Both the MDA prepared according to the invention and the polyisocyanateproduced using this MDA thus displayed substantially improvedproperties.

EXAMPLE 3

MDA was prepared in an apparatus as shown in FIG. 2 and as described inExample 1. The reactor had a volume of 45 m³. The storage vessel 2 wascharged with a mixture of 17,130 kg of aniline and 5378 kg of 30%strength aqueous hydrochloric acid which was then transferred to reactor1. The agitator speed was 70 rpm. The reaction mixture was agitated inthe circuit 3. The circulation rate of the reaction mixture in thecircuit 3 was 300 m³/h. At a reaction mixture temperature of 40° C., inthe course of 60 min at a constant metering rate, in total 6620 kg of a50% strength solution of formaldehyde in water were added to the circuitvia a mixing nozzle as mixing element 6. The reaction mixture was heatedvia a heat exchanger 9. After complete addition of the formaldehyde, thereaction mixture was heated to 90° C. and then charged into a storagevessel having a volume of 70 m³. From this storage vessel, the reactionmixture was transferred via a heating device, with which a reactionmixture temperature of 130° C. was set, into a tubular reactor. Thedwell time in the tubular reactor was 150 min. The mixture was thenneutralized at 103° C. with 50% strength aqueous sodium hydroxidesolution and the organic phase was separated from the aqueous phase. Toremove inorganic impurities, the organic phase was washed with water at95° C. and separated from the aqueous phase. Excess aniline was removedfrom the organic phase in a three-stage distillation at from 180 to 240°C. and a pressure of from 1050 to 5 mbar.

The resultant MDA had an N-methyl MDA content of 0.09% by weight, andthe hydrolyzable chlorine content was 0.04 ppm. The MDA was then reactedwith phosgene at 80° C. and a pressure of 1.5 bar in a stirred-tankcascade having a dwell time of 60 min. The molar ratio of MDA tophosgene was 1:5.2. The phosgenation was carried out in the presence of15% by weight of monochlorobenzene, based on the total weight of thereaction mixture. After the phosgenation, HCl and phosgene were removedat 138° C. and a pressure of 1.2 bar, and then solvent and, ifappropriate, chlorine compounds, were separated off at 180° C. and apressure of 70 mbar. The resulting crude MDI was separated bydistillation at a pressure of 6 mbar and a temperature of 217° C. intopolymeric MDI (PMDI) and monoemric MDI (2,2′-MDI, 2,4′-MDI and4,4′-MDI). The PMDI produced had a hydrolyzable chlorine content of 400ppm and an iodine color index of 10 at a dilution of 1:5 inmonochlorobenzene.

We claim:
 1. A process for preparing methylenedianiline comprisingreacting aniline with formaldehyde in the presence of an acid catalyst,further comprising, in a semicontinuous process, introducing saidaniline with or without said acid catalyst, feeding formaldehyde with orwithout acid catalyst through a mixing element into a circuit in whichaniline with or without acid catalyst and with or without previouslyadded formaldehyde is circulated and, after feeding in at least 50% ofthe total amount of formaldehyde to be fed in, heating the reactionmixture to a temperature above 75° C.
 2. A process as claimed in claim1, wherein the formaldehyde is added up to an amount of at least 50% ofthe total amount of formaldehyde to be added at a temperature of thereaction mixture in the circuit of from 20 to 75° C.
 3. A process asclaimed in claim 1 or 2, wherein the molar ratio of aniline to acidcatalyst is from 1:0.6 to 1:0.01.
 4. A process as claimed in claim 1,wherein the molar ratio of aniline to the total amount of formaldehydeto be added is from 1.7:1 to 7.2:1.
 5. A process as claimed in claim 1,wherein the formaldehyde is fed into the circuit via a nozzle or areaction mixing pump.
 6. A process as claimed in claim 1, wherein thereaction is carried out in an apparatus which has 1: feed lines foraniline and acid catalyst, 2: feed line for formaldehyde, 3: at leastone reaction mixing pump or nozzle through which the formaldehyde is fedinto the apparatus, 4: at least one reactor with or without 5: devicesfor mixing the reaction mixture, 6: a pipe system which, starting fromthe reactor, makes circulation of the reaction mixture possible, 7: adevice for heating the reaction mixture and 8: an optional pump whichcirculates the reaction mixture in (6) and 9: at least one connectionfor taking off the reaction mixture.
 7. A process for preparingpolyisocyanates by phosgenation of methylenedianiline, prepared byreacting aniline with formaldehyde in the presence of acid catalysts,further comprising, in a semicontinuous process, introducing saidaniline with or without said acid catalyst, feeding formaldehyde with orwithout acid catalyst through a mixing element into a circuit in whichaniline with or without acid catalyst and with or without previouslyadded formaldehyde is circulated and, after feeding in at least 50% ofthe total amount of formaldehyde to be fed in, heating the reactionmixture to a temperature above 75° C.
 8. A process for preparingmethylenebis(phenyl isocyanate), MDI, as claimed in claim 7, whichcomprises phosgenating methylenedianiline at a temperature of from 50 to150° C. and a pressure of from 0.5 to 10 bar, in the presence or absenceof inert solvents.
 9. A process as claimed in claim 8, wherein the crudeMDI prepared by phosgenation is purified in such a manner that phosgeneand optionally solvent are removed in a first purification step and thenthe desired monomeric MDI is separated off by distillation and/or bycrystallization.
 10. A process for preparing methylenebis(phenylisocyanate), comprising, in a semicontinuous process, introducinganiline and acid catalyst, the molar ratio of aniline to acid catalystbeing from 1:0.6 to 1:0.01, feeding formaldehyde through a nozzle or areaction mixture pump into a circuit in which said aniline and said acidcatalyst with or without previously added formaldehyde is circulated ata temperature of from 20 to 75° C., after feeding in at least 50% of thetotal amount of formaldehyde to be fed in, heating the reaction mixturefor a period of at least 0.2 hours at a temperature above 75° C., themolar ratio of the aniline introduced to the total amount offormaldehyde to be added being from 1.7:1 to 7.2:1, neutralizing theresulting methylenedianiline, separating off water and aniline,phosgenating the purified methylenedianiline at a temperature of from 50to 150° C. and a pressure of from 0.5 to 10 bar in the presence orabsence of inert solvents, removing phosgene, HCl and optionally solventfrom the crude MDI at a temperature below 150° C. possibly under vacuumor feeding in inert gas, then separating off said solvent from theisocyanate at a temperature of ≦190° C. and then separating off 2,2′-,2,4′- and/or 4,4′-MDI and/or mixtures comprising at least two of theseisomers by distillation at pressures of from 2 to 50 mbar andtemperatures of from 150 to 250° C. and/or by crystallization.
 11. Aprocess for preparing monomeric MDI and polymeric MDI comprising crudeMDI, wherein, in a semicontinuous process, aniline and acid catalyst areintroduced, the ratio of said aniline to said acid catalyst being from1:0.6 to 1:0.01, formaldehyde is fed through a nozzle or a reactionmixing pump into a circuit in which said aniline and said acid catalystare circulated at a temperature of from 20 to 75° C., after the completeaddition of the formaldehyde the reaction mixture is transferred to astorage vessel, heating for a period of from 0.1 to 120 min at atemperature of from 65 to 100° C., then the reaction mixture is heatedin a reactor for a period of from 0.2 to 48 hours at a temperature from105 to 150° C., the reaction mixture is neutralized at a temperature offrom 60 to 110° C., the aqueous phase is separated off by phaseseparation, unreacted aniline is separated off from the organic phase bydistillation, the purified monomeric MDA and polymeric MDA comprisingcrude MDA is phosgenated in the presence of an inert solvent at atemperature from 50 to 150° C. and a pressure of from 0.5 to 10 bar,phosgene, HCl and if appropriate solvent is removed from the processproduct, the crude MDI, at a temperature of from 50 to 149° C. and thensolvent and, if present, chlorine compounds are separated off at atemperature of from 150 to 209° C.
 12. A process for preparing polymericMDI, wherein crude MDI is prepared according to claim 11 and monomericMDI is separated off from the crude MDI by distillation at pressures offrom 2 to 50 mbar and temperatures of from 150 to 250° C.
 13. A processfor preparing monomeric MDI, wherein crude MDI is prepared according toclaim 11 and the monomeric MDI is separated off from the crude MDI bydistillation at pressures of from 2 to 50 mbar and temperatures of from150 to 250° C.